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1、抖动和眼图的视觉化分析 什么是抖动？ TIE 为信号相对于标准时钟或者标准信号的定时误差 TIE 在高速数字系统中即为抖动 抖动为实际数据与其理想位置的时间偏差 P2P3P4P1 0.0ns0.990ns2.000ns2.980ns4.000ns -0.010ns0.000ns-0.020nsTIE0.000ns 什么是眼图？ 眼图是怎么形成的？ Random Jitter(随机抖动随机抖动) 随机抖动符合高斯型分布 直方图(估计) pdf(数学模型) 抖动峰峰值=无穷大无界！ 1-sigma or RMS 7- sigma 内部热能现象 Flicker Noise, Shot Noise 热
2、能的原子与分子振动 分子的解体 外部的宇宙射线 Deterministic Jitter(确定性抖动确定性抖动) 确定性抖动是非高斯分布并且有界 Peak-to-Peak Periodic Jitter(周期性抖动周期性抖动) TIE随时间的变化是重复的、周期性的 Periodic jitter和相位调制(PM)是等效的 Peak-to-Peak 系统时钟（抖动频率在MHz量级） 开关电源（抖动频率在KHz量级） Duty Cycle distortion(占空比失真占空比失真) 上升时间和下降时间不对称 或者测试时参考电平选择不当 0.0v -0.1v Inter-Symbol Interf
3、erence(码间干扰抖动码间干扰抖动) DDJ 或 PDJ 数据相关性抖动或码型相关性抖动, 和ISI的术语是等价的. 码型是如何影响随后的比特位的？ 由于传输链路的效应、反射等 换个角度看抖动，时域 看看我们有了什么视角？ 7/7/201611 抖动视觉化 时间趋势图 直方图告诉了我们分布，但是只有统计特性，缺少了时间信息 时间趋势图可以直观告诉我们波形里是否有特定频率的调制 下图为5个周期SSC 30khz 抖动视觉化 Gaussian Random Noise Sinusoidal Jitter 7/7/201612 7/7/201613 抖动视觉化 频谱图 从频域上观测抖动 抖动中决
4、定性的频率成分会在谱线上明显超出噪底 哪个眼图好？哪个直方图好？ 视觉化眼图和抖动的问题？ 浴盆曲线浴盆曲线 误码率是关键误码率是关键 vs. UI张开程度张开程度 For a given position in the time theres a given probability of error “BER”, Bit Error Ratio For a given position in the time theres a given probability of signal crossing PDF, probability density function 1 UI Probabi
5、lity of hit Probability of Error BER 基于示波器分析的浴盆曲线基于示波器分析的浴盆曲线 Rj/Dj与与Tj BER Assume bi-modal distribution (dual-Dirac), measure Tj at two BER Fit curve to points, slope is Rj, Intercept is Dj Measured Tj 10-7 Measured Tj 10-4 DjxRj Estimated Tj 10-12 x7.4 x10.4 x14.1 双狄拉克模型 Conditions: only where Gau
6、ssian. 抖动类型分析抖动类型分析 抖动分离为误码产生的根本原因提供了更精确的定位和分析方法 抖动分析方法，参照T11 MJSQ，已经被工业界广泛接受 Constituent Components of Jitter = Unbounded = Bounded Total Jitter (TJ) Duty-Cycle Jitter (DCD) Data Dependent Jitter (DDJ) Periodic Jitter (PJ) Deterministic Jitter (DJ) Random Jitter (RJ) 7/7/201618 Jitter Visualization
7、 Bathtub Plot Shows the Eye Opening at a Specified BER Level Note the eye closure of System I vs. System II due to the RJ- RJ is unbounded so the closure increases as BER level increases System I has .053UI of RJ with no PJ System II has .018UI of RJ and .14UI of PJ 5 and 10Mhz System ISystem I Syst
8、em IISystem II Tektronix - Innovators of Jitter Analysis 1998 First Real-Time Scope Based Jitter Analysis Software 2002 Invented SW Based PLL Clock Recovery and the Spectral Approach for Jitter Separation 2004 Invented RT Eye rendering on a Real Time Scope 2004 - First vendor to support both modeled
9、 (Dual-Dirac) and measured (Spectral) jitter methods 2005 - Invented measurements with Jitter and Noise reconciliation 2011 - First scope vendor with BUJ support 2015 RT Noise Analysis and Sampling BER and PDF Mask Testing 抖动和眼图的视觉化 眼图怎么切割的？时钟决定！ TIE 抖动需要参考时钟 参考时钟提取的过程就是时钟恢复 参考时钟有几种确定的方式: Constant C
10、lock with Minimum Mean Squared Error This is the mathematically “ideal” clock But, only applicable when post-processing a finite-length waveform Best for showing very-low-frequency effects Also shows very-low-frequency effects of scopes timebase Phase Locked Loop (e.g. Golden PLL) Tracks low-frequen
11、cy jitter (e.g. clock drift) Models “real world” clock recovery circuits very well Explicit Clock The clock is not recovered, but is directly probed Explicit Clock (Subrate) The clock is directly probed, but must be multiplied up by some integral factor 7/7/201621 Importance of Clock Recovery From s
12、pec, “The jitter measurement device shall comply with the JTF”. How do I verify JTF? JTF is difference between input clock (ref) and input clock (unfiltered) Use 1100b or 0011b pattern (proper 50% transition density) Check 1) LF attenuation, 2) -3 dB corner frequency, and 3) slope 7/7/201622 JTF vs
13、PLL Loop Bandwidth Configuring the correct PLL settings is key to correct measurements Most standards have a reference/defined CR setup For example, USB 3.0 uses a Type II with JTF of 4.9Mhz Type I PLL Type I PLL has 20dB of roll off per decade JTF and PLL Loop Bandwidth are Equal Type 2 PLL Type II
14、 PLL has 40dB of roll off per decade JTF and PLL Loop Bandwidth are not Equal For example, USB 3.0 uses a Type 2 PLL with a JTF of 4.9Mhz. The corresponding loop bandwidth is 10.126 Mhz Setting the Loop Bandwidth as opposed to JTF will lead to incorrect jitter measurement results 7/7/2016 23 PLL Loo
15、p Bandwidth vs. Jitter Transfer Function (JTF) A: Constant Clock Recovery B: PLL Clock Recovery Ratio of B/A 7/7/2016 24 JTF Filtering Effects based on different PLL bandwidths 7/7/201627 f3dB= 30 kHz f3dB= 300 kHzf3dB= 3 MHz Jitter for Busy People Hints, Tips and Common Errors Using the Jitter Analysis Tools Issues manifested in different layers of the protocol stack Crosstalk, jitter, reflections, skew Disparity, encoding or CRC errors Where do I start debugging? Jitter and Eye Diagram Tools Oscilloscope-based for quick resul
